1. Field of the Invention
This invention relates to a semiconductor laser device useful in the fields of optical telecommunication, optical amplification and optical instrumentation as a source of light having a wavelength in a band between about 0.9 and 1.6 .mu.m, more specifically between 0.98 and 1.55 .mu.m, and to a method of making such a semiconductor laser device.
2. Description of the Related Art
One of the biggest problems, from the reliability point of view, with semiconductor laser devices, particularly 0.98 .mu.m laser diodes, is that they have low catastrophic optical damage COD levels. COD is destruction due to light absorption at the laser facet or surface. The exact mechanism of COD is not well understood, but it is generally accepted to be as follows: ##STR1## However, the question remains as to how light absorption starts at the facet in the first place. A normal semiconductor has energy bands bending at the surface as shown in FIG. 1. Using a strain layer InGaAs lattice matched to a GaAs semiconductor substrate will provide a stress relief at the facet, which probably gives rise to bandgap narrowing at the facet as shown in FIG. 2. That is, in the bulk the strain is biaxial and at the facet the strain becomes uniaxial causing bandgap narrowing. Therefore, adding the two effects together the bandgap near the facet becomes as shown in FIG. 3, i.e., Eg.sub.2 &gt;Eg.sub.1. Such an effect should give rise to considerable increase in light absorption and, according to the COD mechanism shown above, will cause the COD level to decrease considerably.
Several ways have been proposed to overcome the problem of COD, including:
(a) re-growth of wideband material such as AlGaAs on a GaAs/InGaAs/AlGaAs laser device, which has been called a "window structure;" PA1 (b) coating the facet with Al.sub.2 O.sub.3 where Al.sub.2 O.sub.3 has a higher thermal conductivity than SiO.sub.2 ; PA1 (c) making a current blocking structure at the facet, in which no current is injected near the facet region; PA1 (d) making a bent waveguide in which the light from one facet comes out from a clad layer instead of the active region; PA1 (e) making a flare structure, in which the beam coming out from one facet spreads to give lower optical density at the facet; and PA1 (f) making a window structure in which zinc is used to create a window.
However, semiconductor laser devices made by each of these methods suffers from significant manufacturing and/or use shortcomings. Method (a) is tedious, and it is difficult and expensive to produce a laser device by this method. Method (b) does not completely get rid of the COD problem, although it is an improvement over laser devices using SiO.sub.2 /Si facet coatings. Method (c) works up to a point in that COD level becomes high before burn-in, however, the COD level undesirably comes down after burn-in. Method (d) is difficult to practice reproducibly. Method (e) also works up to a point in that the COD level is high before burn-in but comes down after burn-in. However, method (e) is better than method (c). Method (f) seems to be the most successful from the viewpoint of long term reliability, but it cannot be used for an InGaAs/GaAs/InGaP material system laser device as In from InGaP gets kicked out into InGaAs on diffusion of zinc.
Of all the methods listed above, only method (f) makes a physical change in the composition of the semiconductor laser device material layers; the rest of the methods simply make geometrical changes to the laser device structure. It must be noted that in methods (a) and (f) a facet "window" is created, but differently. In method (a), the as-grown active region remains and the window (AlGaAs) is grown on the facet. In method (f), the window at the facet is created by zinc diffusion into the active region to cause its bandgap to decrease except near the facet where the bandgap remains as it was grown.